The present invention relates to an input signal processing apparatus for use in a comparator which compares an input signal with a reference voltage signal to output a signal turned or inverted in accordance with a comparison result, and more particularly to such an input signal processing apparatus which relatively biases the input signal and the reference voltage signal in a direction corresponding to a varying direction of the input signal until a predetermined mask time is elapsed after the inversion of the output signal from the comparator.
Generally, for wave-shaping a periodically varying input signal such as a detection signal of a magnet pickup coil used as a rotational speed detecting device, a comparator is used which compares the input signal with a reference voltage signal to decide whether or not the input signal is higher than the reference voltage signal, so as to generate a pulse signal in accordance with the decision result. Here, in such a comparator there is the possibility that the output varies due to introduction of noises into the input signal. Accordingly, as exemplified by the Japanese Patent Publication No. 1-18604, such a comparator is equipped with an input signal processing apparatus whereby the input signal and the reference voltage signal are relatively biased along a direction depending on the varying direction of the input signal for a predetermined time period after the inversion of the output from the comparator so as to prevent a misdecision due to noises. Further, in the case of relatively biasing the input signal and the reference voltage signal after the inversion of the comparator output, if that time period (mask time) is fixed, when the frequency (input frequency) of the input signal is high, the mask time becomes longer than the inversion period of the input signal and hence the input signal and output signal of the comparator becomes different in phase from each other. On the other hand, when the input frequency becomes low, the mask time becomes shorter than the inversion period of the input signal. These cases cause the misdecision to tend to occur due to noises introduced after the elapse of the mask time. Thus, for example, as described in U.S. Pat. No. 4,549,099 (the Japanese Patent Provisional Publication No. 58-188923), the mask time is arranged to vary in accordance with the input frequency, thereby always providing a high noise-removing characteristic and a high phase characteristic.
In the case that, under the condition that the mask time varies in accordance with the input frequency, the frequency of the input signal such as a signal from a rotational speed sensor for detecting as a high rotational speed as several 10 to 10000 rpm widely varies, difficulty is encountered to match the input frequency with the mask time throughout the entire frequency range. In a conventional apparatus, as illustrated in FIG. 8A, in a range in which the frequency of the input signal is high (f&gt;fmax), the mask time TM results in taking the saturated state. Thus, in the high-frequency range (f&gt;fmax), the mask time TM becomes longer than the inversion period of the input signal, whereby there is a problem that the difference in phase between the input signal and output signal of the comparator occurs. That is, in the case that as illustrated in FIGS. 9A and 9B the reference voltage signal VTH is relatively biased or varied with respect to the input signal VIN to have a triangular configuration at every inversion of the comparator output VHL, when the input signal VIN is in a frequency range (f.ltoreq.fmax) that the mask time TM can be set in accordance with the input frequency, it is possible to coincide the phase of the input signal VIN with the phase of the output signal VHL as illustrated in FIG. 9A, and on the other hand, when the input signal VIN is in a high-frequency range (f&gt;f1) that the mask time TM cannot be set in accordance with the input frequency, the input signal VIN and the output signal VHL becomes different in phase from each other as illustrated in FIG. 9B. The phase shift amounts .DELTA.t1 and .DELTA.t2 become greater as the input frequency is higher.
One possible solution is to use a circuit which is capable of setting the mask time in accordance with the input frequency even if the input frequency widely varies. However, difficulty is countered to realize such a circuit in practice. That is, for example, for setting the mask time TM, a frequency-voltage converter (f/V converter) can be used where the inclination of its f/V conversion characteristic is small as indicated by a dotted line in FIG. 8B, thereby preventing the saturation of the mask time until reaching a high-frequency range. However, in this case, the mask time cannot be set accurately due to the error in the f/V converter, and hence there is the possibility that the phase shaft occurs in both the high-frequency range and low-frequency range. In addition, the use of the f/V converter having a high accuracy increases the cost of the apparatus. As a result, for preventing the increase in the cost of the input signal processing apparatus, use of an f/V converter having an f/V conversion characteristic that the mask time takes the saturated state in a high-frequency range as indicated by a solid line in FIG. 8B may be forced.